Scale matching between polyethylene microplastics and soil pores drives nonlinear greenhouse gas emission responses in karst soils

A laboratory microcosm experiment evaluated the effects of polyethylene microplastics (PE-MPs) with different particle sizes on the physicochemical properties, enzyme activities, pore structure, bacterial community, and greenhouse gas emissions of karst soil. The effects of PE-MPs on greenhouse gas emissions were nonlinear and depended on the match between microplastic particle size and soil pore scale. Small-sized particles (10-25 μm) may have filled micropores, induced localized anaerobic microsites, and increased microbial attachment interfaces, potentially enhancing CO₂ and N₂O emissions. Large-sized particles (900-1150 μm) promoted CO₂ emissions, potentially by altering soil pore structure and improving aeration. Medium-sized particles (260-300 μm), being close to the characteristic pore size of the soil, may have caused limited disturbance to pore connectivity and microbial habitats and had no significant effect on either gas. Further analyses indicated that the effects of PE-MPs on greenhouse gas emissions were unlikely to result from direct mineralization, decomposition, or chemical release. Instead, PE-MPs may have indirectly affected greenhouse gas emissions by altering soil physical structure, reshaping bacterial community composition, regulating nitrogen forms, and changing enzyme activities. Small-sized particles exhibited the highest short-term integrated warming effect, followed by large-sized particles, whereas medium-sized particles showed the lowest effect.